Forty-one years ago this Tuesday our moon was first visited by a small shiny craft that descended like a falling star onto the frozen lava plains of the Sea of Tranquility. Apollo astronauts Neil Armstrong and Buzz Aldrin then climbed out and walked the surface of a world untouched for over 4 billion years.

Let's fast-forward to an imaginary time much later in this century. A similarly spidery craft descends to the Sea of Tranquility, but to the viewer's amazement it flies down into a gaping hole on the moon's surface, like a bee going into a hive.

When life was just emerging on Earth, streams of molten lava flowed across the moon and then solidified. Such tubes on Earth form when lava from a volcano starts to cool and form a hardened crust. Hot lava underneath continues flowing in channels.

The moon's frozen tunnels could provide a natural shelter from the extremes on the lunar surface. Assuming we decide to return to the moon someday - it's not on NASA's agenda for now - the lava tubes would allow for ant farm-like colonies of humans living underground.

The tunnels would shield colonists from micrometeorites, lethal X-ray blasts from our petulant sun, and cosmic rays from the galaxy. Temperatures inside the tubes would remain a constant -35 degrees Fahrenheit. That's chilly, but much more stable for complex equipment. On the surface, machinery and structures would degrade under temperature extremes that swing from +250 degrees to -250 during the lunar day/night cycle.

The Hadean fantasy landscape that would await the first lunar spelunkers would be straight out of a science fiction tale. Some tubes may be filled with frozen lava while other may present complex labyrinths.

A big enough skylight would allow automated cargo ships to gingerly descend to the lava tube floor for easy offloading at Moonport 1. A combination of nuclear and solar power generators would keep the colonists toasty. Lighting would cycle to follow a circadian rhythm to keep colonists in sync with Earth's 24-hour day (vs. the moon's two-week-long day).

These would provide the same luxuries to colonists as lunar lava tube bases. Mars pioneers might end up discovering native microorganisms inside the caves. But at least no giant spider-bats as in the cheesy 1959 sci-fi film, "The Angry Red Planet."

Underground cities on Earth were the staple of pulp science fiction. My favorite is the vast subterranean Utopian city in the 1936 H.G. Wells scripted film "Things To Come." (Today it has an uncanny resemblance to the Marriott Marquis hotel in downtown Atlanta - except that the 47-story hotel is definitely above ground!)

Japan's Kaguya (Selene) orbiter first found three pit craters, each the diameter of three football fields end-to-end. The LRO team has added 10 candidate craters - one, ironically enough, in the Sea of Tranquility. (Tranquility Place here, vacationers welcomed!)

The team is planning to acquire stereo images of both large and smaller lava tube skylights now being discovered. The detailed topographic images will better confirm the origin of these unusual features.